1. Field of the Invention
This invention relates to a liquid crystal display device capable of improving a display quality and a driving method thereof.
2. Discussion of the Related Art
A liquid crystal display (LCD) device controls light transmittance of liquid crystal cells in accordance with data signals applied thereto to thereby display an image. In particular, an active matrix type LCD device includes a switching device for each liquid crystal cell. A thin film transistor (TFT) is typically employed as the switching devices for active matrix type LCD devices. Active matrix type LCD devices have various applications, such as monitors for computers and displays for office equipment and cellular phones.
FIG. 1 is a schematic block diagram showing an apparatus for driving a liquid crystal display device according to the related art.
Referring to FIG. 1, the related art liquid crystal display device includes a liquid crystal display panel 2 where m×n number of liquid crystal cells Clc are arranged in a matrix with m number of data lines D1 to Dm crossing n number of gate lines G1 to Gn; a TFT formed near each of the crossings of a data line and a gate line; a data driver 4 for supplying a data signal to the data lines D1 to Dm; a gate driver 6 for supplying a scan signal to the gate lines G1 to Gn; and a timing controller 8 for controlling the data driver 4 and the gate driver 6.
The liquid crystal display panel 2 includes a plurality of liquid crystal cells Clc arranged in a matrix defined by crossings between the data lines D1 to Dm and the gate lines G1 to Gn. The thin film transistor TFT provided at each liquid crystal cell Clc applies a data signal from the data lines D1 to Dm to the liquid crystal cell Clc in response to a scanning signal from the gate lines G1 to Gn. Further, each liquid crystal cell Clc is provided with a storage capacitor Cst for maintaining the voltage of the liquid crystal cell Clc. The storage capacitor Cst may be formed between the pre-stage gate line and the pixel electrode of the liquid crystal cell Clc or between a common electrode line and the pixel electrode of a liquid crystal cell Clc.
The gate driver 6 sequentially applies a scanning pulse to the gate lines G1 to Gn in response to a control signal CS from the timing controller 8 to thereby select horizontal lines of the liquid crystal display panel 2 to be supplied with the data signals.
The data driver 4 converts digital video data R, G and B into analog gamma voltages, i.e., data signals, corresponding to gray level values of the digital video data in response to the control signal CS from the timing controller 8, and applies the analog gamma voltages to the data lines D1 to Dm.
The timing controller 8 generates the control signal CS for controlling the gate driver 6 and the data driver 4 using synchronizing signals and a clock signal supplied from an external source. The control signal CS for controlling the gate driver 6 includes a gate start pulse GSP, a gate shift clock GSC, and a gate output signal GOE. The control signal CS for controlling the data driver 4 includes a source start pulse SSP, a source shift clock SSC, a source output enable SOC, and a polarity signal POL. The timing controller 8 re-arranges the data Data supplied from the exterior to supply to the data driver 4.
In the related art liquid crystal display device, the liquid crystal cells on the liquid crystal display panel 2 are driven using an inversion driving method such as a frame inversion system, a column inversion system, or a dot inversion system. In the driving method of the frame inversion system, the polarity of the data signals supplied to the liquid crystal cells on the liquid crystal display panel 2 is inverted whenever a frame is changed. In the driving method of the line inversion system, the polarity of the data signals supplied to the liquid crystal cells is inverted for each horizontal row of the liquid crystal display panel 2. In the driving method of the column inversion system, the polarity of the data signals supplied to the liquid crystal cells is inverted for each column of the liquid crystal display panel 2. The dot inversion system has a pixel voltage signal supplied to each given liquid crystal cell having a polarity opposite that of the polarity supplied to the liquid crystal cells that are adjacent to the given liquid crystal cell on the liquid crystal display panel 2 in the vertical and horizontal directions. Further, in the dot inversion system, the polarity of the pixel signals supplied to the liquid crystal cells on the liquid crystal display panel 2 is inverted for every frame.
Among these inversion driving methods, the dot inversion system provides the best picture quality as compared with the frame inversion system, the line inversion system and the column inversion system. The driving of the dot inversion system is performed by controlling the data signals from the data driver 4 in response to the polarity signal POL supplied from the timing controller 8.
Typically, the liquid crystal display device is driven by a frame frequency of 60 Hz. However, a lower frame frequency in the range of 30˜50 Hz may be used in systems such as a notebook computers to lower power consumption. As the frame frequency is lowered, a phenomenon where the display presents a greenish color occurs even in the dot inversion system. A horizontal 2-dot inversion system has been proposed for operation at lower frame frequencies as a solution to this greening problem.
Referring to FIG. 2A and FIG. 2B, the horizontal 2-dot inversion system is set to change the polarity every two liquid crystal cells in a horizontal direction and in addition, to invert the liquid crystal cells vertically adjacent to have a polarity different from each other. Further, the polarity of the liquid crystal cells is inverted every frame. The driving method of horizontal 2-dot invention system provides improved picture quality as compared with other inversion methods at lower frame frequencies.
However, in the horizontal 2-dot inversion system, a dominant polarity is generated when displaying images having a specific repeating pattern. The dominant polarity leads to a problem wherein a voltage of a common electrode Vcom is changed.
More specifically, as shown in FIG. 3, when a specific pattern is repeated, the polarity of the liquid crystal cells of a horizontal line is differently set. In other words, in the (+) polarity dominant lines, 8 liquid crystal cells are supplied with positive polarity data signals and 4 liquid crystal cells are supplied with negative polarity data signals. On the other hand, in the (−) polarity dominant lines, 8 liquid crystal cells are supplied with negative polarity data signals and 4 liquid crystal cells are supplied with positive polarity data signals. When a dominant polarity of positive polarity or negative polarity data signals is generated for each line, the voltage of the common electrode is changed causing a brightness deviation between the liquid crystal cells. Accordingly, a smear phenomenon such as crosstalk occurs, thereby lowering display quality.